The present invention relates to DC-to-AC convertors for electric power systems. Lower cost, high-power, efficient, DC-to-AC convertors are of interest for solar energy economics. In the prior art, DC-AC inverters are the second highest cost item next to the photovoltaic panels. For high efficiency and low heat dissipation, commutation of DC to produce AC preferably uses solid state switches that are either fully on or fully off, and do not dwell more than a microsecond or so in an intermediate state. Therefore it is more complicated to produce a sine wave that takes on all values between the negative peak and the positive peak. On the other hand, producing a square wave which switches between the positive peak and the negative peak produces a form of AC that is not suitable for all loads.
Various manufacturers provide prior art DC-AC convertors that fall into one of a few broad classes and operating modes. The class of “modified sine wave” converters maintains both the same rms and the same peak voltage as a sine wave, while still employing only on-off commutation. This is done by switching the voltage between the desired positive peak, zero and the negative peak, spending 50% of the repetition period at zero, therefore achieving both the same peak and the same rms values as a true sine wave, and being compatible with a greater variety of loads.
Still, there are loads that do not tolerate the modified sine wave; for example appliances that present inductive loads, such as induction motors, some cellphone and laptop battery chargers, fluorescent lamps and tumble dryers, and any device with an internal power supply that uses capacitive reactance as a lossless voltage-dropping means, can malfunction on modified sine waveforms. Moreover, there is a potential problem with radio and TV interference due to the high level of harmonics of the modified square wave converter. Such a waveform is therefore not a candidate for coupling solar-generated power into the utility network or into house wiring.
“True sine wave” is another class of prior art DC-AC converter, and is required for coupling power into the grid or into premises wiring.
Another categorization of convertor relates to whether they are designed to power loads directly, or whether they are designed to feed and sell power back into the electricity grid. A load inverter that can power loads directly is said to operate in standalone mode, and is also called a “standalone inverter”, while a grid-tie inverter is said to operate in grid-interactive mode and is also called a “grid-interactive inverter”.
For safety and other reasons, the latter have to meet different specifications than the former, especially under fault conditions. In particular, a load inverter should be a constant voltage source, while a grid-tie inverter does not have a constant voltage output but must adapt to the voltage of the grid, and is rather a controlled current source. Moreover, a load inverter is always used with energy storage such as a rechargeable battery, and should maintain efficiency at both light and heavy loads and have low, no-load power consumption, so that the battery is not discharged while the inverter is idling at night. Grid-tie inverters however do not have the same a requirement for no-load power consumption, as they do not operate at night.
Many prior art inverters used low-frequency transformers in the synthesis of sine waveforms, but the large amount of copper and iron required for low-frequency transformers adds significant cost and weight.
Transformerless inverters are known in the prior art, particularly for utility-interactive inverters, which use high-frequency switching or pulse width modulation to approximate a sine wave. However, a disadvantage that arises in certain of these these converter concepts is the imposition of the high-frequency switching waveform on the solar array, which can capacitively couple through the glass cover upon touching it, potentially causing RF burn to personnel or damage to the solar panel, as well as causing the solar array to radiate substantial radio interference. Thus a design is required that can create a more benign common-mode voltage fluctuation on the solar array DC conductors.
One known method of making grid-tie inverters to convert DC power from a solar array to AC power than can be back fed into the grid is to employ multiple microinverters connected to the grid in parallel. This arrangement has been pioneered by, for example Enphase Inc.
When microinverters are attached to each solar panel, the advantage is the elimination of DC wiring, for which the National Electrical Code has specified new, unusual and onerous regulations.
However, multiple microinverters are more costly than a single large inverter. There is therefore the desire to reduce the cost of multiple microinverters to equal or better the cost of a single large inverter while retaining the benefit of eliminating DC wiring inside the premises.